1. Field of the Invention
The present invention relates to photoelectric conversion devices.
2. Description of the Related Art
A number of photoelectric conversion devices used for detecting an electromagnetic wave are generally known, and for example, photoelectric conversion devices having sensitivity from ultra violet rays to infrared rays are collectively referred to as optical sensors. An optical sensor having sensitivity to light in a visible light region with a wavelength of 400 nm to 700 nm is particularly referred to as a visible light sensor, and a large number of visible light sensors are used for devices which require illuminance adjustment, on or off control, or the like depending on human living environment.
In some display devices, ambient brightness of the display device is detected to adjust display luminance. This is because with optical sensors, visibility can be improved or wasted power consumption can be reduced by detecting ambient brightness and obtaining appropriate display luminance. For example, as display devices which include optical sensors for adjusting luminance, mobile phones and computers can be given. In addition, as well as the ambient brightness of a display portion, luminance of the backlight of a display device, in particular, a liquid crystal display device is detected by an optical sensor to adjust luminance of a display screen.
When weak light to strong light are detected in an optical sensor, the range of photocurrent is expanded; therefore, output voltage increases linearly proportional to illuminance in the case where the photocurrent is converted into voltage. Accordingly, when output voltage is obtained in a wide illuminance range, several millivolts are obtained in the case of weak light, and several volts are obtained in the case of strong light. Thus, due to limitations on a circuit (e.g., power supply voltage), it is difficult to broaden the dynamic range of illuminance as an optical sensor. Patent Document 1 discloses an optical sensor in which a wider range of illuminance can be detected without expansion of the range of output voltage or output current.
A logarithmic output illuminance sensor which can obtain output voltage proportional to a logarithmic value of illuminance has been developed because power consumption can be reduced. In addition, spectral sensitivity characteristics required for the illuminance sensor are necessary to be made closer to the eye-spectral sensitivity. Thus, a photodiode and the like mounted on such an illuminance sensor are often formed with a material which has low sensitivity in regions other than a visible light region.